This invention relates to turbomolecular vacuum pumps and hybrid vacuum pumps and, more particularly, to vacuum pumps having impeller configurations which assist in achieving one or more of compact pump structures, increased discharge pressure and decreased operating power in comparison with prior art vacuum pumps.
Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial pumping stages and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high rotational speed by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages.
Variations of the conventional turbomolecular vacuum pump often referred to as hybrid vacuum pumps, have been disclosed in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages, which form a molecular drag compressor. This configuration is disclosed in Varian, Inc. U.S. Pat. No. 5,238,362, issued Aug. 24, 1993. Varian, Inc sells hybrid vacuum pumps including an axial turbomolecular compressor and a molecular drag compressor in a common housing. Molecular drag stages and regenerative stages for hybrid vacuum pumps are disclosed in Varian, Inc. U.S. Pat. No. 5,358,373, issued Oct. 25, 1994. A gradual change in the design of the stators of the axial pumping stages is also disclosed in U.S. Pat. No. 5,358,373. Other hybrid vacuum pumps are disclosed in German Patent No. 3,919,529, published Jan. 18, 1990; U.S. Pat. No. 5,848,873, issued Dec. 15, 1998; and U.S. Pat. No. 6,135,709, issued Oct. 24, 2000. The disclosed hybrid vacuum pumps use existing impeller types and switch abruptly from one impeller type to another.
Molecular drag stages include a rotating disk, or impeller, and a stator. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. As is known in the art, the momentum of the rotating disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. Molecular drag stages were developed for molecular flow conditions.
Another type of molecular drag stage includes a cylindrical drum that rotates within a housing having a cylindrical interior wall in close proximity to the rotating drum. The outer surface of the cylindrical drum or the wall is provided with a helical groove. As the drum rotates, gas is pumped through the groove by molecular drag.
A regenerative vacuum pumping stage includes a regenerative impeller, which operates within a stator that defines a tangential flow channel. The regenerative impeller includes a rotating disk having spaced-apart radial ribs at or near its outer periphery. Regenerative vacuum pumping stages were developed for viscous flow conditions.
In molecular flow, pumping action can be produced by a fast moving flat surface dragging molecules in the direction of movement. Depending on design, very high-pressure ratios per stage can be achieved by a single disk impeller having a flat surface.
When viscous flow is approached, the simple momentum transfer does not work as well, because of increased backward flow due to the establishment of a pressure gradient rather than a molecular density gradient. At the high end of the pressure spectrum, there is a well-known art of regenerative stages or blowers, which, near atmospheric pressure, produce pressure ratios more than two per stage at high peripheral velocities.
However, the impellers for molecular drag stages and the impellers for regenerative blowers do not work efficiently throughout the pressure range involved in high vacuum pumps. Flat surface impellers work reasonably well at pressures up to about one torr in medium sized pumps. Above that pressure level, flat surface impellers become inefficient and begin to require excessive power and produce unwanted heat, as well as exhibiting a reduction in the achievable compression ratio. Attempts to extend the flat surface design to atmospheric pressure have not been successful because of the need for very small gaps between moving and stationary surfaces. Regenerative blowers work best above about 20 torr, producing satisfactory pressure ratios. Usually a particular design produces a narrow range of efficient operation. Therefore, the design of impellers is important with respect to power saving in order to reduce heating of the rotor.
Hybrid vacuum pumps, which utilize molecular drag stages typically, have rotor-stator gaps of about eight thousandths of an inch. Reducing the gap to smaller than this dimension requires extremely tight tolerances and increases cost. This gap dimension necessitates a relatively large number of stages to achieve the desired overall compression ratio. However, [these] this approach results in increased cost and size, and may require an unacceptably long rotor shaft.
Accordingly, there is a need for vacuum pumps having impeller configurations, which overcome one or more of the above disadvantages.
According to a first aspect of the invention, a vacuum pump is provided. The vacuum pump comprises a housing having an inlet port and an exhaust port, a plurality of vacuum pumping stages located within the housing and disposed between the inlet port and the exhaust port, and a motor. The vacuum pumping stages comprise molecular and transition flow drag stages, each including a stator and an impeller. The impellers of successive ones of the gas drag stages are configured for efficient operation at progressively higher pressures. The motor rotates the impellers such that gas is pumped from the inlet port to the exhaust port.
The gas drag stages may include a first stage wherein the impeller comprises a disk having a smooth pumping surface and a second stage wherein the impeller comprises a disk having a roughened pumping surface. The gas drag stages may further include a third stage wherein the impeller comprises a disk having a grooved pumping surface. The vacuum pumping stages may further comprise one or more regenerative stages.
The impellers of successive ones of the molecular drag stages may have pumping surfaces with a surface topography for efficient operation at progressively higher pressures. The pumping surface of the impeller may be an annular region at or near the outer periphery of the disk. The pumping surface may include all or part of the front surface, all or part of the rear surface and/or all or part of the edge surface of the impeller.